A variety of multimedia services in wireless environments are required due to the rapid growth of the mobile communication market. Large-capacity data must be transmitted at a high speed in order to provide the multimedia services. Thus, research is being conducted on a multiple antenna system (e.g., a Multiple Input, Multiple Output (MIMO) system) for efficient use of limited frequency resources.
MIMO schemes for the multiple antenna system can be classified into an open-loop MIMO scheme and a closed-loop MIMO scheme depending on whether a transmitting side detects channel information. In the open-loop MIMO scheme, a transmitting side transmits data without knowing channel information. Examples of the open-loop MIMO scheme include a Space-Time Coding (STC) scheme and a Vertical-Bell Labs lAyered Space-Time (V-BLAST) scheme. In the closed-loop MIMO scheme, a transmitting side acquires channel information and transmits data using the acquired channel information. Examples of the closed-loop MIMO scheme include a Singular Value Decomposition (SVD) scheme and a Space Division Multiple Access (SDMA) scheme.
When the multiple antenna system uses the closed-loop MIMO scheme, a method for acquiring channel information by a transmitting side varies depending on the duplexing schemes of the multiple antenna system. For example, when the multiple antenna system uses a Frequency Division Duplex (FDD) scheme, a receiving side measures a channel through a signal received from a transmitting side and feeds the measured channel value back to the transmitting side. Thus, the transmitting side can acquire channel information. On the other hand, when the multiple antenna system uses a Time division Duplex (TDD) scheme, each of receiving sides transmits a channel sounding signal for discrimination between the receiving sides to a transmitting side. Then the transmitting side measures an uplink channel through the channel sounding signal and uses the measured uplink channel value as information about a downlink channel. This is based on the channel reciprocity property that the uplink and downlink channels are equal to each other.
In the multiple antenna system, a base station and a user terminal each have a plurality of antennas. The user terminal can receive signals through a plurality of receiving (RX) antennas, thereby achieving a diversity gain. Also, the base station can transmit a plurality of streams to a user terminal through a plurality of transmitting (TX) antennas, thus increasing the data transmission rate of the user terminal. However, in general, the user terminal uses only one TX antenna in a TX mode because it has a limited battery life and thus must transmit a signal at a much lower TX power than the base station. That is, the user terminal uses all of the antennas in an RX mode but uses only one of the antennas in a TX mode. Thus, when the multiple antenna system uses the closed-loop MIMO scheme, a user terminal transmits a channel sounding signal to a base station through only one TX antenna. In this case, the base station cannot acquire information about the entire downlink channel for the user terminal. This may reduce the data reception rate of the user terminal.